Membranes having selective permeability are classified according to the size of substances to be separated. For example, membranes for liquid treatment are generally classified into ultrafiltration and microfiltration membranes for separating substances such as colloids and proteins; nanofiltration membranes for separating low-molecular-weight organic substances such as agricultural chemicals; and reverse osmosis membranes for separating ions. Reverse osmosis membranes are used at pressures higher than the osmotic pressure of the liquid to be treated, and at pressures of several MPa in the case of seawater desalination.
Examples of the shapes of membranes having selective permeability include flat sheet membranes, tubular membranes, spiral wound membranes, and hollow fiber membranes. Among these examples, hollow fiber membranes provide a large membrane area per unit volume of membrane module, and are therefore suitable for membrane separation processes, thus finding wide application, for example, in the area of seawater desalination with reverse osmosis membranes. A hollow fiber membrane module typically comprises a pressure vessel, a hollow fiber membrane element, permeated fluid collectors for collecting the permeated fluid flowing through a plurality of hollow fiber membranes, and other components. It is particularly necessary that the hollow fiber membrane element and the permeated fluid collector be in close contact with each other. In the present invention, a hollow fiber membrane element having permeated fluid collectors closely affixed thereto is referred to as a hollow fiber membrane submodule.
Such a hollow fiber membrane submodule is useful when a plurality of hollow fiber membrane elements, particularly hollow fiber membrane elements of the type in which both ends open, are installed in a single pressure vessel, because it facilitates extraction of the permeated fluid from the open ends of the hollow fiber membranes.
In conventional hollow fiber membrane submodules, permeated fluid collectors are attached to a hollow fiber membrane element by bonding to achieve simple configuration and easy manufacture. For example, Japanese Unexamined Patent Publication No. 1997-511447 (Japanese translation of WO9608306) discloses a hollow fiber membrane submodule configuration comprising hollow fiber membranes both of whose ends are open, and permeated fluid collectors (end caps) attached to both open ends.
However, a problem with this hollow fiber membrane submodule is that because the permeated fluid collectors are attached to the hollow fiber membrane element by bonding or the like, the entire hollow fiber membrane submodule has to be replaced when replacing the membrane. Accordingly, the permeated fluid collectors are also disposed and replaced, and thus cannot be reused, which is disadvantageous in terms of ecology and cost efficiency. Moreover, because the permeated fluid collectors are not removable, if the submodule proves to be defective after performance evaluation, the open surfaces of the hollow fiber membranes cannot be tested and/or examined for leaks, or cannot be repaired. In addition, with a large hollow fiber membrane element, direct installation of permeated fluid collectors or end caps into the element is difficult. Furthermore, upon installation of the element in a pressure vessel, a space must be provided between the inner surface of the pressure vessel and the hollow fiber membrane submodule for the feed or concentrated fluid to pass through, and therefore the outside diameter of the hollow fiber membrane element has to be made small. This results in reduction in the membrane area of the hollow fiber membranes and accompanying reduction in the flow of permeated fluid.
Alternatively, permeated fluid collectors may be fastened to a hollow fiber membrane element using screws. This configuration enables collectors to be removed from and installed in the element. The disadvantage of this configuration, however, is that during membrane treatment operation, the screws may become loose by the vibration of the pump feeding a feed fluid to the hollow fiber membrane submodule. Moreover, although metal is a preferable material for the screws of this configuration, the metal is subjected to corrosion in treating such liquids as seawater, and therefore is not preferable.